This disclosure relates generally to hydraulic valve circuits for use with material handling equipment and, more particularly, to hydraulic valve circuits adapted to control transversely movable members such as forks or clamp arms associated with material handling equipment, and to provide damage-control override capabilities for the control thereof.
Material handling equipment used for moving palletized or non-palletized loads from place to place, such as, for example, in a warehouse, typically includes forklift trucks or other types of vehicles equipped with material handling attachments having load-lifting members such as forks or clamp arms. For example, on a typical forklift truck, load-lifting forks are attached to a carriage which is in turn movably attached to a mast so as to travel vertically for raising and lowering the forks. Various different types of attachments may also be mounted on the carriage. For example, a fork side-shifter which moves the forks transversely in unison, and a fork positioner which moves the forks transversely toward and away from each other, may be attached to the carriage, either separately or as an integrated unit. Alternatively, a load clamp having load-engaging clamp arms similarly movable transversely either in unison, or toward and away from each other, may be attached to the carriage. Such general types of equipment, as well as those more specifically described hereafter, all constitute exemplary applications in which the hydraulic circuits described herein are intended to be used.
Different types of load-lifting forks and clamp arms are available for these purposes. For example, drum-clamping forks may incorporate contours particularly useful for clamping barrels or drums. Similarly, clamp arms may be engineered differently for handling rectangular or cylindrical loads. More specifically, clamp arms adapted to handle rectangular loads such as stacked cartons or household appliances are generally referred to as carton clamps and rely on clamping forces applied to the sides of the rectangular load for lifting the load. Carton clamp attachments typically include a pair of large blade-shaped clamp members each of which can be inserted between side-by-side stacks of cartons or appliances. The clamp members on either side of the load are then drawn together, typically using hydraulic cylinders for controlling the movement of the clamp members, to apply a compressive force on the load of sufficient pressure to allow for lifting the load using the clamp members compressively engaged with the sides of the load. The surfaces of the clamp members which contact the sides of the load are typically constructed of materials such as rubber faced aluminum that provide a high coefficient of friction to securely hold the load. Carton clamps are most frequently used in the warehousing, beverage, appliance, and electronics industries and may be specifically designed for particular types of loads. For example, carton clamps may be equipped with contact pads that are sized for palletless handling of refrigerators, washers, and other large household appliances (also referred to as “white goods”). In various configurations, carton clamps may be used for handling multiple appliances at one time.
In addition to clamping a load in order to lift and move the load, carton clamps may be equipped with side-shifting capabilities whereby the clamped load may be repositioned from side-to-side with the clamping members moving transversely in one direction or the other in unison. The side-shifting function may be actuated by one or more hydraulic cylinders separate from the clamping cylinder (“external” side-shifting), or by the clamping cylinders themselves (“internal” side-shifting). After a particular load is grasped and lifted by the clamping members, moved to a new location, and perhaps side-shifted toward one side or the other, the operator of the forklift or other material handling vehicle fitted with the carton clamp attachment might then lower the load, release the clamp pressure, and thereafter decide that repositioning of the load is needed. For example, the load may need to be repositioned to sit flush against a trailer or railcar wall. To reposition the load, the operator may contact the load with an outward side surface of one of the clamp members and then turn the vehicle toward the load, causing the outward surface of the clamp member to urge the load to move transversely (in a maneuver referred to as “pinwheeling”). Alternatively, the operator may contact the load with an outward side surface of one of the clamp members and then use either a clamp opening movement or a side-shifting movement toward the load to cause the outward surface of the clamp member to reposition the load transversely (in a maneuver referred to as “backhanding”).
Damage to the load may occur in various ways. The operator may use too little clamping force when attempting to grasp and then lift the clamped load. As a result the load may become dislodged from the clamping members and sustain impact damage. A more likely scenario involves the operator using too much clamping force in an effort to avoid dropping the load. The result of using too much clamping force may be a crushed or deformed load.
Damage to the load or to adjacent structures such as surrounding walls or trailers, railcars, containers or warehouses, may also occur if excessive pinwheeling or backhanding force is applied. For example, using too much force to reposition a load against a trailer wall might result in crushing or deforming the load as it is compressed between the outward surface of one of the clamp members and the trailer wall, or deforming or even breaking the trailer wall itself.
To prevent the operator of a material handling vehicle, equipped with any of the foregoing load-handling attachments, from accidentally causing such damage to a load or to an adjacent structure, hydraulic override valve circuits are needed that are adapted to limit the compressive forces that may be applied to the side of a load by forks, clamp arms, or similar load-handling members, not only to limit clamping forces but also to limit pinwheeling or backhanding forces which may be applied to the side of the load when repositioning the load.